Microbiology - Chapter 5 - Control of Microbial Growth

Physical methods

Include heat treatment, irradiation, filtration, and mechanical removal (washing).

Chemical method

Use any of a variety of antimicrobial chemicals.

Sterilization

The process of removing or destroying all microorganisms and viruses on or in a product. Can be achieved through filtration, heat, chemicals and irradiation.

Sterile

Absolutely free of microbes, including endospores and viruses.

Sterile (does not include)

Prions.

Disinfection

Is the process that eliminates most or all pathogens on or in a material. Generally, this term implies the use of antimicrobial chemicals.

Disinfectants

Are those used for disinfecting inanimate objects.

Disinfectants (also called)

Biocides, germicides.

Antiseptics

Is when disinfectants are formulated to use on skin. Used on living tissues.

Pasteurization

Uses a brief heat treatment to reduce the number of spoilage organisms and kill pathogens. Food and inanimate objects can be pasteurized. Surfaces can also be pasteurized.

Decontamination

Is a treatment used to reduce the number of pathogens to a level considered safe to handle. The treatment can be as simple as thorough washing, or it may involve the use of heat or disinfectants.

Degerming

Is a treatment used to decrease the number of microbes in an area, particularly the skin. This is not a specific level of control

Sanitized

Generally implies a substantially reduced microbial population that meets accepted health standards.

Preservation

Is the process of delaying spoilage of foods or other perishable products. Often includes the addition of growth-inhibiting ingredients

Nosocomial

contracted as a result of being hospitalized; hospital-acquired

Bacterial endospores (characteristic)

The endospores of Bacillus and Clostridium (and related genera) are the most resistant form of life typically encountered. Only extreme heat or chemical treatment ensures their complete destruction.

Protozoan cysts and oocysts (characteristic)

Cysts and oocysts are stages in the life cycle of certain intestinal protozoan pathogens such as Giardia lamblia and Cryptosporidium parvum. These disinfectant-resistant forms are excreted in the feces of infected animals, including humans, and can cause diarrheal disease if ingested. They are concerned in water treatment and readily destroyed by boiling.

Mycobacterium

Cell wall structure initiates resistance to chemical. Thus stronger, more toxic disinfectants must be used to disinfect environments that may contain Mycobacterium tuberculosis, the causative agent of tuberculosis.

Pseudomonas species

Can grow in presence of many chemical disinfectants. Common cause of infection in hospital.

Naked viruses

Lack envelope and are more resistant to chemical killing

Decimal reduction time (D value)

Is the time required to kill 90% of population under specific conditions. Washing reduces time required to reach disinfection or sterilization.

Most chemicals are more effective at higher temperatures and lower pH (T or F)

True.

Effectiveness can be hampered by the presence of organism molecules (T or F)

True.

Critical instrument

Come into direct contact with body tissues, including needles, scalpels, and biopsy forceps.

Semicritical instruments

Come into contact with mucous membranes, but do not penetrate body tissue, including gastrointestinal endoscopes and endotracheal tubes.

Non-critical instruments

Poses little risk of infection because they only come into contact with unbroken skin. Countertops, stethoscopes, and blood pressure cuffs are examples of non-critical items.

Moist heat

Denatures proteins. Relatively fast, reliable, safe and inexpensive. Widely used.

Boiling

Boiling for 5 minutes destroys most microorganisms and viruses; a notable exception is endospores. Boiling for at least 5 minutes can be used to treat drinking water.

Pasteurization

Significantly decreases the number of heat-sensitive microorganisms, including spoilage microbes an dpathogens (except sporeformers). Milk is pasteurized by heating it to 72C for 15 seconds. Juices are also routinely pasteurized.

Pressurized steam (autoclaving)

Typical treatment is 121C/15 psi for 15 minutes or longer, a process that destroys endospores. Heated water -> steam -> increased pressure. Preferred method of sterilization. Flash autoclaving sterilizes at 135°C and 15psi in 3 minutes. Prions destroyed at 132°C and 15psi for 4.5 hours

Dry heat

Not as effective as moist heat. Sterilization requires longer times and higher temperatures. 200°C for 1.5 hours vs. 121°C for 15 minutes.

Incineration

Oxidizes cell to ashes. Used to destroy medical waste and animal carcasses. Flaming laboratory inoculation loop incinerates organism ->Results in sterile loop

Dry heat ovens

Oxidizes cell components and denatures proteins. Less efficient than moist heat, requiring longer times and higher temperatures.

Filtration

Filter retains microbes while letting the suspending fluid or air pass through small holes.

Filtration of fluids

Used for heat sensitive fluids. Membrane filters allow liquids to flow through. Traps microbes on filter. Depth filters trap microbes using electrical charge.

Filtration of air

HEPA filters are used to remove microbes that have a diameter greater than 0.3 micrometer. Used in biological safety cabinets, specialized hospital rooms, and airplanes. Also used in some vacuum cleaners and home air purification units.

Radiation

Type of cell damage depends on the wavelength of the radiation.

Ionizing radiation

Radiation able to strip electrons from atoms. Comes from 3 sources, Gamma radiation, X-rays, Electron accelerators. Causes damage to DNA and potentially to plasma membrane. Used to sterilize heat resistant materials, medical equipment, surgical supplies, medications. Some endospores can be resistant

Ultraviolet radiatioin

Damages DNA, penetrates poorly. Only type to destroy microbes directly. Used to destroy microbes in air, drinking water and surfaces. Limitation: poor penetrating power, thin films or coverings can limit effect

High pressure processing

Used in pasteurization of commercial foods. Does not use high temperatures. Employs high pressure up to 130,000 psi. Destroys microbes by denaturing proteins and altering cell membrane permeability

FDA regulates

Antiseptics

EPA regulates

Disinfectants

Sterilants

Can destroy all microorganisms, including endospores and viruses.

High level disinfectants

Destroys viruses and vegetative cells, not endospores

Intermediate-level disinfectants

Kills vegetative cells fungi, most viruses, not endospores

Low-level disinfectants

Removes fungi, vegetative bacteria and enveloped viruses, not mycobacteria, naked viruses or endospores.

Toxicity

Benefits must be weighed against risk of use

Activity in presence of organic material

Many germicides inactivated in presence of organic matter.

Compatibility with material being treated

Liquids cannot be used on electrical equipment

Residue

Residues can be toxic or corrosive

Storage and stability

Concentrated stock relieves some storage issues

Environmental risk

Is germicidal agent harmful to environment

Germicides classes

Alcohols, Aldehydes, Biguanides, Ethylene oxide, Halogens, Metals, Ozone, Peroxides, Phenolics, Quaternary ammonium compounds

Alcohols (ethanol and isopropanol)

Solutions of 60% - 80% isopropyl or ethyl alcohol kill vegetative bacteria and fungi, not effective against endospores and some naked viruses. Coagulation of proteins and essential enzymes, damage to lipid membranes. Commonly used as antiseptic and disinfectant. Limitations ->1) Evaporates quickly limiting contact time. 2) May damage material such as rubber and some plastics

Aldehyedes (glutaraldehyde, orthophthaladehyde, and formaldehyde)

Capable of destroying all forms of microbial life. Irritating to the respiratory tract, skin, and eyes. Formalin used to kill bacteria and inactivate viruses, also used for specimen preservation, formalin is solution made from formaldehyde. Glutaraldehyde and ortophthalaldehyde are used to sterilize medical instruments.

Biguanides (chlorhexidine)

Most effective member of group is chlorhexidine. Extensively used in antiseptics, relative low toxicity, destroys wide range of organisms.

Ethylene oxide

Easily penetrates hard-to-reach places and fabrics and does not damage moisture-sensitive material. It is toxic, explosive, and potentially carcinogenic. Commonly used to sterilize medical devices.

Halogens (chlorine and iodine)

Common disinfectants. Mode of action ->Oxidizing proteins and other cell components. Chlorine -> Destroys all types of organisms and viruses, used as disinfectant ->Caustic to skin and mucous membranes. Chlorine dioxide replacing chlorine in many applications. Iodine ->Kills vegetative cells, not reliable, with endospores, used in tincture or iodophore on skin.

Metal compounds

most are too toxic to be used medically. Silver sulfadiazine is used in topical dressings to prevent infection of burns. Silver nitrate drops can be used to prevent eye infections caused by Neisseria gonorrhoeae in newborns. Some metal compounds are used to prevent microbial growth in industrial processes.

Ozone

O3, Unstable form of oxygen. Powerful oxidizing agent. Used as alternative to chlorine, as disinfectant for drinking and waste water

Peroxygens

Includes hydrogen peroxide and peracetic acid, powerful oxidizing agents, readily biodegradable, less toxic than ethylene oxide and glutaraldehyde.

Hydrogen peroxide

Effectiveness depends on surface being treated ->Living tissue produce catalase enzyme, which breaks down hydrogen peroxide to oxygen and water. Thus hydrogen peroxide is more effective on inanimate object. Useful as disinfectant because it lLeaves no residue and doesn't damage most materials. Hot solutions used in food industry. Vapor-phase can be used as sterilant

Peracetic acid

More potent then hydrogen peroxide, and is effective on organic material. Can be used to disinfect and sterilize medical devices.

Phenolics compounds (triclosan and hexachlorophene)

Destroy plasma membrane and denature proteins. Active ingredient in Lysol. Wide range of activity, reasonable cost, remains effective in the presence of detergents and organic contaminants, leaves an active antimicrobial residue. Triclosan is used in a variety of personal care products, including toothpastes, lotions, and deodorant soaps. Hexachlorophene is highly effective against Staphylococcus aureus, but its use is limited because it can cause neurological damage.

Quaternary ammonium compounds (benzalkonium chloride and cetylpyridinium chloride)

A.k.a Quats. Cationic detergents. Nontoxic. Used to disinfect food preparation surfaces. Mode of action ->Reduces surface tension: Aids in removal of dirt and organic matter, facilitates mechanical removal of organisms. Positive charge attracts Quats to negative charge of cell surface ->Reacts with membrane, destroys vegetative bacteria and enveloped viruses, not effective on endospores, mycobacteria and naked viruses

Chemical preservatives

Numerous chemicals are used as preservatives, formaldehyde, Quats, and phenols. Weak organic acids often used as food preservatives, Benzoic, ascorbic and propionic acids. Used in bread, cheese and juice. Mode of action ->Alter cell membrane function, interfere with energy transformation. Nitrates and nitrites used in processed meats ->1) Inhibits germination of endospores and growth of vegetative cells, 2) Have been shown to be potent carcinogen

Low temperature storage

Microbial growth is temperature dependent -> 1)Low temperatures slow down or stop enzymatic reactions of mesophiles and thermophiles, some psychrophiles still able to grow. Freezing means of food preservation -> 1) Essentially stops microbial growth. 2)Irreversibly damages cell ->Kills up to 50% of microbes, Reaming cells still pose potential threat

Reducing water availability

Decreasing water availability accomplished by salting or drying food. 1) Addition of salt increases environmental solutes, causes cellular plasmolysis. 2) Numerous bacteria can continue to grow in high salt environments -> Staphylococcus aureus can survive in high salt concentrations. 3) Desiccation or drying is often supplemented by other methods ->Salting. 4) Lyophilization (freeze drying) -> Widely used to preserve foods like coffee, milk and meats